Add errdefs package

Providers should use this package so the virtual kubelet core
controllers can understand the errors produced from the provider code.

errdefs/wrapped.go

@@ -0,0 +1,10 @@
+package errdefs
+
+// Causal is an error interface for errors which have wrapped another error
+// in a non-opaque way.
+//
+// This pattern is used by github.com/pkg/errors
+type causal interface {
+	Cause() error
+	error
+}